Methods and apparatus for manufacturing pulp fibers from lignocellulose-containing materials generally include preheaters for preheating chips or the like. Subsequent to the preheater, defibering apparatus is generally used for slushing and refining of the chips to form pulp while generating steam between two opposed grinding disks which are rotating relative to each other. These grinding disks are generally enclosed in a grinding housing in which an overpressure is generally maintained. The pulp so produced, together with the steam, then generally flows from the grinding housing through a blow pipe to a steam separator. From there, the pulp is fed to a further device for processing of the pulp. Because of the desirability of utilizing the energy content of the steam separated from the pulp, the steam is generally recycled to the preheater for use in heating the chips themselves. The steam separator is normally a vessel in the form of a cyclone in which the steam is removed from the top of the cyclone and the pulp is removed from the bottom of the cyclone by means of a separate, air-tight feeding-out device, such as in the form of a plug-forming screw conveyor.
In these installations a lower pressure is generally maintained in the steam separator than that in the grinding housing. In this manner, the pulp and steam flow from the grinding housing to the separator, and the separated steam can be returned to the preheater by the use of a fan or compressor so that the required steam pressure and corresponding temperature can be maintained in the preheater.
It is also possible in these installations to transfer the material from the preheater to the defibrator by means of a steam-proof conveyor. In this manner, the steam can flow from the grinding housing through the steam separator to the preheater, since the steam pressure in the grinding housing is maintained at a higher level than the pressure in the preheater without steam flowing backwards from the defibrator to the preheater. By means of these arrangements, the energy content in the developed steam can be recovered by the material in the preheater by those chips being preheated to the highest possible temperature. However, in order to do so, the required steam transport must be secured by a fan, compressor, or a steam-proof feeder to the defibrator. This additional equipment is a considerable disadvantage, since it raises the price of the installation and renders it more complicated.